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STRATIGRAPHY AND SEDIMENTOLOGY OF OIL-GAS BASINS 2012 / 2

Stratigraphy

Introduction The Ameki Formation has previously been

noted mainly for its rich content of molluscan fauna (Newton, 1922; White, 1926; Eames, 1957), until Adegoke et al (1980) described its first nautiloid fauna, Deltiodonautilus nwajidei, named after Prof. C.S. Nwajide who collected the holotype while carrying out fieldwork in 1977. The holotype collected has some missing parts and its thickness cannot be determined. Also, only ten of its chambers were preserved and the position of the siphuncle was unknown. However, in 2007, a paratype of this nautiloid was discovered with complete parts. This para-type occurs in association with some fish frag-ments and gastropods embedded in limestone concretions, thin shelled oysters, crustacean fragments and phosphatic nodules. These fos-sils were recovered from an erosional cut around Bende – Idima Abam junction, 500m west of Bende Town (Figure 1), along Bende – Ohafia road. The aim of this paper is to erect a paratype of D. nwajidei, which will shed more light on the holotype especially in interpreting the paleodepositional environment of the Ameki Formation and placing the fossils in their proper sedimentological and paleo-geographic context.

Method Of Study

An outcrop section of the Ameki Formation

located 500 m east of Bende, at Bende – Idima Abam road junction was systematically logged, described and sampled. Materials collected from the site were thoroughly examined. The macro-fossil samples were hand – picked, described and identified using relevant paleontological monographs (Newton, 1922; White, 1926 and Adegoke, 1977).

Geological Setting

The Eocene Ameki Formation is a compo-

nent of the Ameki Group, which also consists of the Nanka Sands and the Nsugbe Sandstone (?). This formation is a lithostratigraphic equivalent

Chukwuemeka Frank Raluchukwu Odumodu

Department of Geology, Anambra State University, P.M.B. 02, Uli, Anambra State

E-mail: raluchukwu@yahoo.com Bruno Ndicho Nfor

Department of Geology, Anambra State

University, P.M.B. 02, Uli, Anambra State E-mail: nforbng@yahoo.com

A NEW PARATYPE OF AN EOCENE NAUTILOID FROM THE AMEKI FORMATION, SOUTHEASTERN NIGERIA: IMPLICATIONS

FOR AGE AND PALEOENVIRONMENTAL INTERPRETATION A large sized paratype of the nautiloid Deltoidonautilus nwajidei was recently recovered

from the basal part of the Ameki Formation at Bende in southeastern Nigeria. Other fauna collected from this location include; fish remains, gastropods and bivalves. The fish remains include chondrichthyans such as Odontaspis koerti, Myliobatis dixoni, Myliobatis toliapicus and Myliobatis bothriodon and the vertebra Cylindricantus rectus. Gastropods include Cytu-lotilibia unidiginata and Planaxis africana; the bivalves include Glycymeris (Glycymeris) togoensis and the crab Callianasa biformis. This faunal content and lithofacies are indicative of middle-to-outer neritic zone of the marine shelf depositional environment during the Mid-dle Eocene time.

Keywords: Ameki Formation, Eocene, Nautiloid, Holotype, Paratype, Marine shelf.

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Chukwuemeka Frank Raluchukwu Odumodu, Bruno Ndicho Nfor A NEW PARATYPE OF AN EOCENE NAUTILOID FROM THE AMEKI FORMATION, SOUTHEASTERN NIGERIA: IMPLICATIONS FOR AGE AND PALEOENVIRONMENTAL INTERPRETATION

of the Eocene Agbada Formation of the Niger Delta Basin. According to Nwajide and Reijers (1996), the sea level rise in the Paleocene termi-nated the filling of the Anambra Basin, while the succeeding regression gave rise to the Niger Delta Basin. The Ameki Formation overlies the Paleocene marine Imo Formation and is over-lain by the paralic Ogwashi – Asaba Formation, which is Oligocene in age.

Sedimentary Facies

The Ameki Formation comprises of sandstones, shales, siltstones, clays, thin limestone bands and nodules, and is about 287m thick (Figure 1) (Arua, 1986; Arua and Rao, 1987). Arua (1986) subdivided it into five sedimentary facies, which include; calcareous sandstone facies, pebbly bio-

turbated sandstone facies, silty shale facies, argil-laceous sandstone facies, and pebbly sandstone facies. The calcareous sandstone facies consists of three subfacies. The study area lies within sub-facies 1 of the calcareous sandstone facies, and consists of an alternation of gray clay – shale and dark gray highly fossiliferous, argillaceous mic-rite and sandy intramicrite. The section studied at Bende – Idima Abam junction, 500 m west of Bende, along Bende – Ohafia road (Figure 2) consists of alternations of thinly laminated grey clayey shale and dark grey highly fossiliferous limestones with septarian and phosphatic no-dules. The basal section is overlain by laminae and thin beds of siltstones, fine sandstones and clay-shales. The adjacent farmlands which lie beneath the basal section are mud cracked and contain scattered boulder-sized septarian nodules.

Figure 1. Geological map and litholog of Ameki Formation showing the lithofacies and the fossil localty (Modified from Arua, 1986)

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STRATIGRAPHY AND SEDIMENTOLOGY OF OIL-GAS BASINS 2012 / 2

Stratigraphy

,

Myliobatis Odontaspis koerti Deltoidonautilus nwajidei

Moluscs Limestone nodules Bivalves

Lithology Descriptions

0

0.5 M

Alternations of thinly laminated parallel laminated gray clayey shale and limestones containing nautiloids, fish remains gastropods, bivalves and crabs.

Interlaminations of siltstones, fine sandstones and clayey shales containing nautiloids, fish fragments, gastropods, bivalves and crustacean fragments

Fauna

Crabs

Paleontology

The faunal elements include nautiloids (Figure 3), fish fragments, gastropods, bivalves and crustacean fragments (Figure 4). The nauti-loid includes one large D. nwajidei and some other fragmentary remains. The fish remains include four chrondrichthyans and one vertebra. The chrondrichthyans include Odotaspis koerti (Stromer), Myliobatis dixoni (Agassiz), Mylio-batis toliapicus (Agassiz) and Myliobatis bothriodon. The vertebrate is identified as Cylindri-canthus rectus. The gastropods present include dwarf and well developed forms of Cytulotilibia unidiginata, Heligmotoma nigeriensis, Norrisa aurilitoralis, Solariella adedayoi and Planaxis africana Adegoke. The bivalves include Glycy-meris (Glycymeris) togoensis, Ostrea amekien-sis and microforms of Mytilus nigeriensis. The crab fragmentary remains belong to the genus Callianassa biformis. The vertical association of the fauna shows that all of them occur together on two horizons while the fish remains occur at different horizons all through the studied section (Figure 2).

Systematic Paleontology Class: CEPHALOPODA, Curvier, 1797 Subclass: NAUTILOIDEA, Agassiz, 1847 Order: NAUTILIDA, Agassiz, 1846

Superfamily: NAUTILACEA, de Blain-ville, 1826

Family: HERCOGLOSSIDAE, Spath, 1927 Species: Deltoidonautilus nwajidei, Arua

and Adegoke, 1977; Adegoke, Arua and Oye-goke, 1980.

Diagnosis: - This is a large sized Deltoi-donautilus with about twenty one whorls. Early whorls are subequal in size but subsequent whorls successively increase in size. Sutures have gentle curves and converge ventrally. The venter is prominently keeled.

Descriptions: - The shell is moderately large, evolute and subdiscoidal in shape. The septum increases in size successively away from the protoconch. The shell surface is marked by sutures characterized by very broad to gently rounded ventral saddle and a broad but mode-rately deep lateral lobe, which converges at the umbilicus. It has a small rounded umbilicus which is closed by a calcareous plug. D. nwa-jidei possesses about twenty one chambers of which the last four appear to be inflated. The siphuncle is located at the lower part of the peristome. The whorl is sub triangular in shape, has an angular venter and a pronounced keel.

Material: - This is a complete, well pre-served specimen.

Dimensions: - Height (mm) Width (mm) Maximum thickness (mm) 133.3 mm 114 mm 74.9 mm

Figure 2. Lithologic section of Ameki Formation, 500 m west of Bende at Bende – Idima Abam Junction

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Chukwuemeka Frank Raluchukwu Odumodu, Bruno Ndicho Nfor A NEW PARATYPE OF AN EOCENE NAUTILOID FROM THE AMEKI FORMATION, SOUTHEASTERN NIGERIA: IMPLICATIONS FOR AGE AND PALEOENVIRONMENTAL INTERPRETATION

Remarks: The paratype of D. nwajidei possesses the diagnostic features of the holotype as outlined by Adegoke et al (1980), who

include its relatively highly inflated body whorl and its prominently keeled venter.

Figure 3a. Ventral view of Deltoidonautilus nwajidei Figure 3b. Lateral view of Deltoidonautilus nwajidei

Figure 4. Some faunal elements and material from the section studied

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STRATIGRAPHY AND SEDIMENTOLOGY OF OIL-GAS BASINS 2012 / 2

Stratigraphy

Discussion Paleoenvironmental Interpretation

Several depositional environments have

been suggested for the Ameki Formation and are based mainly on its faunal content. White (1926) considered the Ameki system as estuarine due to the presence of fish remains of estuarine habitat. Adegoke (1977) contended that the fishes were washed from inland waters into the Ameki Sea. He therefore suggested an open marine environ-ment for the Ameki Formation. Reyment (1965) suggested a depositional environment varying from marine, through partly non-marine to estua-rine. Arua (1986) also opined a depositional en-vironment ranging from near shore (barrier ridge – lagoonal complex) to the subtidal and intertidal zones of the marine shelf.

Evidence from sedimentary facies, struc-tures and faunal content of the studied section very clearly suggests a marine shelf depositional setting for this section, which is most probably the basal section of the Ameki Formation. The alternations of laminas and beds of clayey shales, limestones, siltstones and fine sandstones is sug-gestive of middle and outer neritic shelf. The faunal assemblage, which consists of nautiloids, sharks and rays, gastropods, bivalves and crab fragments of Callianassa species are typical in-habitants of neritic depths offshore – to the inter-tidal environment. The presence of shark teeth suggests that the area has a direct link to the open sea. The phosphorites are typical of the open ma-rine outer shelf / slope setting associated with areas of organic productivity, mostly in low oxy-gen rather than full anoxic conditions, occurring as pellets, nodules, crusts and thin beds (Stow, 2006). Stow (2006) further suggested that they are abundantly formed during periods of high sea level and widespread transgression leading to shallow fertile shelf seas.

Age

Steven et al (2011) assigned a Paleocene

age to the sediments at this section using sedi-

mentological differences between the Imo and Ameki Formations without considering their faunal contents. The faunal diversity at this sec-tion has revealed an assemblage of fossils be-longing to Paleocene and Eocene ages. The nau-tiloids D. nwajidei (Adegoke et al, 1980), the sharks and rays, the gastropod Cytulotilibia unidignata, the bivalve Ostrea amekiensis and the crab Callianasa are typical Eocene fossils. Again, the gastropod Cytulotilibia unidignata is the most abundant fossil recovered from the sec-tion. However, some Paleocene macrofauna such as the gastropod Planaxis africana, Helig-motoma nigeriensis, Norrisa aurilitoralis and Solariela adedayoi, the bivalves Glycymeris (Glycymeris) togoensis and Mytilus nigeriensis range into the section and thus suggest the pres-ence of the Paleocene Eocene boundary some-where beneath the section. It is therefore sug-gested that this section represents the basal part of the Ameki Formation and is Middle Eocene in age and not the entire Paleocene.

Paleogeography

This study suggests that the basal part of the Ameki Formation was deposited in the mid-dle to outer neritic zone of the marine shelf de-positional environment. Figure 5 is a 3-D paleo-geographic model of this part of the Ameki Formation showing the fossils in their sedimen-tological and paleogeographic context. The lithology, sedimentary structures and fossil con-tent very clearly supports the deposition of the sediments in a marine shelf environment.

Conclusions

The paratype of the nautiloid D. nwajidei

was recently recovered from the basal section of the calcareous sandstone facies of the Ameki Formation. This subfacies consists of an alterna-tion of gray clay – shale and dark gray highly fossiliferous, argillaceous micrite and sandy intramicrite. The nautiloid occurs in association with some fish remains, gastropods, bivalves,

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Chukwuemeka Frank Raluchukwu Odumodu, Bruno Ndicho Nfor A NEW PARATYPE OF AN EOCENE NAUTILOID FROM THE AMEKI FORMATION, SOUTHEASTERN NIGERIA: IMPLICATIONS FOR AGE AND PALEOENVIRONMENTAL INTERPRETATION

Imo Formation

Ameki Formation

Myliobatis

Odontaspis koerti

Deltoidonautilus nwajidei

Moluscs

Limestone nodules

Shales Siltstones Sandstones

Bivalves

crustacean fragments and phosphorites. The fish remains include chondrichthyans such as Odon-taspis koerti, Myliobatis dixoni, Myliobatis to-liapicus and Myliobatis bothriodon while the

vetebrate is Cylindricantus rectus. The lithofa-cies, sedimentary structures and faunal assem-blage suggest the deposition of the sediments in a marine shelf setting during the Middle Eocene.

REFERENCES

Adegoke, O.S., 1977. Stratigraphy and paleontology of the Ewekoro Formation (Paleocene) of Southwestern Nigeria. Bulls. Amer. Paleont., 71(295), 375 p.

Adegoke, O.S., Arua, I., and Oyegoke, O., 1980. Two new Nautiloid from Imo Shale (Paleocene) and Ameki Formation (Middle Eocene), Anambra Basin, Nigeria. Jour. Min. Geol., 17 (1), pp. 85 – 89.

Arua, I., 1986. Paleoenvironment of Eocene deposits in the Afikpo syncline, southern Nigeria. Jour. Afr. Earth Sci. 5 (3), pp. 279 – 284.

Arua, I. and Adegoke, O.S., 1977. Additional stratigraphically important nautiloid species from southern Nigeria. Abstracts 6th European Malac, Congr, pp. 136.

Arua, I and Rao, V.R., 1987. New stratigraphic data on the Eocene Ameki Formation, southeastern Nigeria. Jour. Afr. Earth Sci., 6 (4), pp. 391 – 397.

Eames, F.E., 1957. Eocene Mollusca from Nigeria: a revision. Brit. Mus. (Nat. Hist.) Bull. 3, pp. 25 – 70. Newton, R. B., 1922. Eocene Mollusca from Nigeria. Geol. Surv. Nigeria Bull. 31, 115 p. Nwajide C.S., and Reijers, T.J.A., 1996. Anambra basin Excursion Guide. In Reijers, T.J.A. (ed).

Selected Chapters on Geology, Shell Petroleum Company of Nigeria, Corporate reprographic services, Warri, pp. 149 – 197.

Reyment, R.A., 1965. Aspects of the Geology of Nigeria, Ibadan University Press, Nigeria, 145p. Stevens, N.J., Eastman, J.T., Odunze, S.O., Cooper, L.N. and Obi, G.C., 2011. Paleocene

Ichthyofauna and paleoenvironmental setting, Imo Formation, southern Nigeria. Neues Jahrbuch fur Geologie und Palaontologie – Abhandlungen, 260 (3), pp. 289 – 296.

Stow, D.A.V., 2006. Sedimentary rocks in the field, A colour guide. Manson Publishing Limited. London, 297 p.

White, E. I., 1926. Eocene fishes from Nigeria. Geol. Surv. Nigeria Bull, 10, 82 p.

Figure 5. 3-D Paleogeographic model of the Ameki Formation

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STRATIGRAPHY AND SEDIMENTOLOGY OF OIL-GAS BASINS 2012 / 2

Stratigraphy

CƏNUB-ŞƏRQİ NİGERİYA AMEKİ FORMASİYASINDAN EOSEN NAUTİLOİDLƏRİNİN YENİ PARATİPİ: STRATİQRAFİQ VƏ PALEOEKOLOJİ İNTERPRETASİYALARDA TƏTBİQ

Çukvumeka Frank Raluçukvu Odumodu, Bruno Ndixo Nfor

Böyük ölçülü Deltoidonautilus nwajidei nautiloidləri cənub – şərqi Nigeriyada Bende

rayonunda Ameki formasiyasının bazal hissəsində bu yaxınlarda tapılmışdır. Başqa fauna qalıqlarına balıq qalıqları, qarınayaqlı və ikitaylı molyuskların qabıqları daxildir. Balıq qalıqlarından Odontaspis koerti, Myliobatis dixoni, Myliobatis toliapicus və Myliobatis bothriodon, o cümlədən Cylindricantus rectus müəyyən edilmişdir. Qarınayaqlılar içərisində Cytulotilibia unidiginata və Planaxis african, ikitaylılardan isə Glycymeris (Glycymeris) togoensis, həmçinin krablardan Callianasa biformis tapılmışdır.

Belə fauna kompleksi və çöküntülərin litofasial tərkibi, tədqiqat aparılan rayonda Eosen zamanı dəniz şelfinin orta – xarici zonasının mövcud olduğunu göstərir.

НОВЫЙ ПАРАТИП ЭОЦЕНОВЫХ НАУТИЛОИД ИЗ ФОРМАЦИИ АМЕКИ, ЮГО-ВОСТОЧНАЯ НИГЕРИЯ: ПРИМЕНЕНИЕ В СТРАТИГРАФИЧЕСКИХ

И ПАЛЕОЭКОЛОГИЧЕСКИХ ИНТЕРПРЕТАЦИЯХ

Чуквумека Франк Ралучукву Одумоду, Бруно Ндихо Нфор

Большого размера наутилоиды Deltoidonautilus nwajidei были недавно найдены в ба-зальной части формации Амеки в районе Бенде в Юго – восточной Нигерии. Другие фау-нистические находки включают рыбные остатки, раковины гастропод и двустворчатых моллюсков. В рыбных остатках определены Odontaspis koerti, Myliobatis dixoni, Myliobatis toliapicus и Myliobatis bothriodon, среди которых найдены остатки Cylindricantus rectus. В составе гастропод обнаружены Cytulotilibia unidiginata и Planaxis african; а бивальвий – Glycymeris (Glycymeris) togoensis. Также найден краб Callianasa biformis.

Подобный фаунистический комплекс и литофациальный состав указывают на су-ществование в районе исследований в эоценовое время средней – внешней зоны морского шельфа.